Display screen [of] or portion thereof with a transitional graphical user interface



FIG. 1 is a front view of a display screen or portion thereof with a transitional graphical user interface, showing our new design and depicting a first image thereof; and,

FIG. 2 is a front view of a display screen or portion thereof with a transitional graphical user interface, showing our new design and depicting a second image thereof.

The appearance of the transitional graphical user interface sequentially transitions through FIGS. 1-2 in a first embodiment, and through FIGS. 2-1 in a second embodiment. The process or period in which one image transitions to another image forms no part of the claimed design.

The innermost rounded rectangular outline rendered in solid lines shows part of the display screen and forms part of the claimed design. All other broken lines show other parts of the display screen and form no part of the claimed design. 

CLAIM The ornamental design for a display screen or portion thereof with a transitional graphical user interface, as shown and described. 